Studded tires or snow chains were used for driving on snowy and icy roads; however, they unfortunately cause environmental problems, such as dust pollution. Studless winter tires were thus developed as alternative tires for driving on snowy and icy roads. Since studless winter tires are for use on snowy roads with rougher surfaces than normal roads, the materials and structure thereof are specially designed. For example, a rubber composition that contains a diene rubber having excellent low-temperature properties, and a rubber composition that contains a large amount of softener to enhance the softening effect have been developed (for example, Patent Literature 1).
Some attempts have been made to enhance the performance on ice and snow of studless winter tires, for example, by increasing the softener content in the rubber composition. However, too high a softener content may cause the tread rubber to undergo a greater change in hardness with age (hardening degradation), which may reduce the performance on ice and snow with age (hereinafter, also referred to as performance on ice and snow after degradation). Thus, the softener content cannot be increased unlimitedly. Moreover, since the initial performance on ice and snow and the resistance to change in hardness with age are opposed to each other, it has been hard to simultaneously enhance the “performance on ice and snow after degradation” and the “initial performance on ice and snow.”